Glioblastoma multiforme (GBM) is the most common subtype of primary brain tumors in adults. GB is very aggressive, highly invasive, and infiltrates critical brain structures. Mean survival time from the time of diagnosis is 6-12 months, in spite of advances in chemotherapy, surgery and radiotherapy. Harnessing the host's immune system to develop novel treatments for GBM has been attempted. Systemic immunization against glioma has not yielded beneficial effects, since it is limited by progressive tumor mutation, and a brain micro-environment not conducive to sustaining immune responses. Priming immune responses from within the brain parenchyma itself is limited by the paucity of dendritic cells (DC) and an immune-suppressive environment. We have tested in a large syngeneic rodent intracranial glioma model a combined gene therapy approach using first generation recombinant adenovirus vectors expressing fms-like tyrosine kinase 3 ligand (FltSL) (to recruit DCs to the brain), and herpes simplex virus type 1 thymidine kinase (HSV1-TK) that through the phosphorylation of the prodrug GCV (GCV) induces tumor cell death, making potentially antigenic tumor epitopes available to the DCs recruited to the brain. This approach was successful in eradicating a large rodent macroscopic tumor, for which all other single gene therapies tested failed. However, in humans who may have been exposed to adenovirus earlier in life, the immune response can completely abolish expression from first generation adenovirus. Therefore, we have now shown that this can be overcome by the use of the novel high capacity adenoviral vectors (HC-Ad). For this U01 proposal, we will perform experiments to stringently assess the efficiency and potential neuropathological, systemic, or behavioral side effects of the treatment of brain tumors with HC-Ad expressing FltSL in combination with HC- Ad- HSV1-TK+GCV. Our long term aim is to move this gene therapy approach towards Phase I clinical trials. Following a stringent timeline with specific milestones, we propose to test our HC-Ad approach first in a large rodent glioma for safety and efficacy, secondly in endogenous spontaneous dog GBMs for efficacy, safety, systemic biodistribution, and unforeseen side effects, and, finally, test safety and biodistribution in the brain of a non-human primate model. Having met all previous milestones, we propose to file for FDA approval towards the end of this U01 proposal.